Due Date Calculator
Estimate your due date and see how many weeks pregnant you are today.
About this tool
Uses Naegele’s rule: 280 days (40 weeks) from the first day of your last period, adjusted if your cycle is longer or shorter than 28 days, or 266 days from conception. It is an estimate for information only and does not replace advice from your midwife or doctor.
Pregnancy is usually dated from the first day of your last menstrual period, and a baby is considered due 40 weeks later. Enter that date, or your conception date if you know it, to estimate your due date, see how many weeks and days pregnant you are today and which trimester you are in. It runs in your browser, so the dates you enter are not stored or sent anywhere.
How to use the Due Date Calculator
- Choose whether to calculate from your last period or from the conception date.
- Enter the date.
- If you are using your last period, enter your usual cycle length; 28 days is average.
- Read your estimated due date, how far along you are and your trimester.
Last period on 1 March 2026
With a 28-day cycle, adding 280 days to 1 March 2026 gives an estimated due date of 6 December 2026, and conception most likely happened around 15 March. With a 32-day cycle, ovulation tends to happen about four days later, so the due date moves to 10 December. On 1 June 2026 you would be 13 weeks pregnant, just finishing the first trimester.
How due dates are worked out
Naegele’s rule counts 280 days, or 40 weeks, from the first day of the last period. It assumes a 28-day cycle with ovulation on about day 14, which is why the first two weeks of a pregnancy happen before conception. If your cycles are longer or shorter, the date is moved by the difference. From a known conception date, such as with IVF, the count is 266 days. Pregnancy is split into three trimesters: weeks 0 to 13, 14 to 27 and 28 to birth. A due date is only an estimate. Only a small share of babies arrive on the day itself, and birth anywhere from 37 to 42 weeks is common. An early ultrasound scan measures the baby and is usually the most accurate way to date a pregnancy, so your care team may give you a different date.
Frequently asked questions
How accurate is a due date calculator?
It gives the standard estimate used by doctors and midwives, but only about 4% of babies are born on their due date. Most arrive within two weeks either side.
Why does pregnancy start before conception?
By convention, pregnancy is counted from the first day of your last period, about two weeks before you ovulate and conceive. That is why a full-term pregnancy is called 40 weeks.
What if my cycles are irregular?
A calculation from your last period is less reliable. An early ultrasound scan gives a more accurate date, so ask your doctor or midwife.
When does each trimester start?
The first trimester runs to the end of week 13, the second from week 14 to the end of week 27, and the third from week 28 until birth.